Technical Data and Techniques
At the University, our aim is to develop knowledge and techniques. This page contains the results and analysis of some of the development work that has been carried out, principally on hybrid motors. As work is carried out here, data and technical papers will be added so that the information is shared for the benefit of rocketeers.
Student projects 2008-9
Students this year are working on ignition methods for hybrid motors, measurement of combustion-chamber and oxidiser tank pressures in hybrid motors, a non-pyrotechnic ejection system for rockets and the development of a small hybrid motor to production standard. The reports will be posted on this site when available. An MSc project has been proposed for a feasibility study of a CO2-based hybrid motor - this has already been developed in the USA for a lab-based example. We'd like to build a flight-capable motor, but it's a challenging undertaking and may take more than one project to achieve.
Student projects 2007-8
We have a number of students who have finished rocketry-based projects, and the outcomes may be of interest to rocketeers. The projects include visualisation of the filling of a hybrid motor nitrous-oxide tank, flow from the injector, a cross-flow nitrous injector and its performance, and a comparison of metal-loaded hybrid fuel grains with unloaded grains. In particular, the behaviour of nitrous oxide as it fills and empties from the tank is surprising, and to our knowledge has not been seen before. The reports are being made available as we get them.
Comparison of Metal-Loaded and Non-Loaded Hybrid Fuel Grains
This project involved making up HTPB fuel grains with and without powdered aluminium, and comparing their performance. Although only a small number of firings were achieved, the results were quite interesting. It is also interesting to note that the regression rate of HTPB grains is significantly higher than plastic grains.
See the report by Richard Tosh ( 2124Kb)
Flow Visualisation of the Oxidiser Injection in a Hybrid Rocket Motor
Since nitrous oxide is semi-cryogenic, its behaviour during filling, and also when it emerges from the injector during firing, differs from an ordinary liquid. Jason used a couple of transparent rigs to look at both of these phases of operation. The report shows the nitrous oxide in a simulated flight tank during the filling and dumping (roughly equivalent to firing) and shows how it behaves like both a liquid and a gas. The report also shows how the nitrous oxide enters the combustion chamber, and the distances taken to atomise and vaporise.
See the report by Jason McHugh (2126Kb)
Design, Manufacture and Test of a Reduced-Length Hybrid Rocket Motor
Similar to our 'shorty' hybrid concept, this student project was to design, manufacture and test a hybrid motor that is much shorter than a conventional hybrid, making it suitable for use in rockets that were not originally designed for them. Mike did exactly that, and has developed his motor to a state where it is about to be put into production. His motor has flown a couple of times under the 'experimental'' rules, and we are now looking to certify it when our test cell has been installed.
See the report by Mike Reynolds (800Kb)
Investigation into the Injector Configuration of a Hybrid Rocket Motor
Most hybrid motors use a fairly crude injector - just the burned-off stump of the fill pipe, meaning that the nitrous oxide is dumped into the combustion chamber as a single stream directly down the fuel-grain port. This project was similar to that performed on the Pentamax motor last year, but this time on a 38mm Sky Ripper J. It used an injector that directed four fine streams of oxidiser outwards and forwards, away from the nozzle, giving the maximum opportunity for vaporisation. The arrangement for the fill pipe was also quite different from the Pentamax project. The result was a different pattern of burning and significantly improved performance.
See the report by Jamie Smith (1430Kb)
Incident report
An incident happened at a demonstration launch, and the following details may be helpful to other rocketeers. A demonstration of a tetrahedron was being prepared, powered by a 2-grain Pro38 H motor. There was a large crowd of over 100 people, who were positioned behind a fence to ensure a safe distance from the launch position. The launch controller was a home-made one, consisting of a pad-side box containing a 12V battery, power switch, 12V relay and a voltmeter. The circuit was arranged such that setting the power switch to 'ON' applied power to the voltmeter and to one side of the relay, as well as the relay contacts. The outgoing connection to the launch controller provided a 12V supply down one wire and returned though the other, such that the launch controller effectively applied a short circuit to trigger the relay. The launch controller was fitted with a safe-arm keyswitch, an LED to show the controller was connected and armed, and two pushbuttons for launch, which need to be pressed together. This design is common to many launch controllers, and is recommended by many as a safe design.
After setting up and testing correct function (by hearing the relay click), the keyswitch was set to 'SAFE' and the launch controller placed on the ground. The power switch on the pad-side box was set to 'OFF', and this was confirmed by checking that the voltmeter display was blank. Person A checked this, and advised Person B that the pad was safe. B was at the pad, connecting the igniter, whilst A went to deliver a safety briefing to the crowd. No-one was near the launch controller. B also checked that the display on the voltmeter was blank, then proceeded to connect the igniter wires. During the delivery of the safety brief, and as the second igniter wire was connected to the clip, the motor ignited and a normal but premature flight ensued. No injuries were sustained by anyone.
The pad-side box was subsequently opened and no faults or stray wires were found. A voltmeter was connected to the clips and all connections were moved, and the leads out to the launch controller were shorted together. It proved impossible to generate any voltage on the igniter clips. The set-up had been used many times before without incident or apparent fault.
Since the incident would have required at least three separate faults in the launch controller setup, unless a stray wire had been the cause, it is thought extremely unlikely that this was the cause, especially given the fault-free history of the equipment. Both rocketeers are very experienced HPR fliers and have also worked together many times. It is suspected that either static electricity or radio-frequency pickup, perhaps from B's mobile phone, was the cause. Although B received no call or message on the phone, it would still be communicating with the nearest transmitter. The weather was hot and humid, of the kind associated with thunderstorms.
It is recommended that rocketeers should touch the igniter clips together briefly immediately before connecting them. This makes a final check that the launch controller has not been inadvertently left live and also will help to disperse static electricity. Both rocketeers already have adopted the habit of connecting clips at arm's length, as far from the rocket as possible, with head and eyes turned away, always assuming that the motor could ignite. This habit is recommended to all fliers. On this occasion, these precautions ensured that this incident did not become an accident.
Student projects 2006-7
Hybrid fuel-grain materials
In 2006-7, we had a student conducting a final-year project on this subject. He compared the performance of around half a dozen different plastics. The work was done using our RATT I-80 motor, but is applicable to all motors. The results are now available - see the link below. In the 2007-8 academic year we are running a similar project using HTPB, with and without loading with powdered metals.
See the report by Allan Palmer (1400Kb)
Injector configurations
The 54mm Pentamax motor uses a five-port injector with 1/8" pipe fittings. Two of the ports can optionally be blanked off to reduce thrust and increase burn time. This project involved investigating various configurations, including impinging injectors, swirl and cross-flow (similar to those available for the large RATT motors), and comparing them with the original design. The results are now available - see the link below.
See the report by Mark Sadler (720Kb)
Parachute shock loads
When selecting a parachute, most rocketeers focus on choosing the right size to match the weight of the rocket, and on price and perhaps appearance. There is another aspect, though, which is how much load the parachute will create on the airframe as it deploys. Are they all the same, or do some create less load than others? Does the method of folding make any difference? This project examined a number of different parachute sizes, types and wind speeds, and tested many combinations in a wind tunnel. The results make interesting reading - not only is the TAC-1 an attractive chute, it brings a rocket down more slowly, all other things being equal, and also creates much lower shock loads as it deploys.
See the report by Abdul (Raf) Khan (845Kb)
Igniting hybrids
There are several methods of igniting hybrids, and the best one depends greatly on the motor in question, especially its size - what is suited to a 29mm diameter motor may be ineffective for a 54mm or bigger motor. The most common methods used currently are AP igniter grains (sometimes referred to as pre-heaters) and gaseous oxygen or GOx. Other methods are used, including plastic initiator cord (pic) and pyrodex pellets.
The situation in the UK is not the same as the US, however, because legislation, and the interpretation of that legislation, is different. For example, until recently it was believed that the cutting of composite propellant grains to make igniter grains was illegal, being regarded as an act of manufacture. More information from the Health and Safety Executive seems to take a different interpretation, and this method is now accepted, although there are still some who have their doubts. The igniter grains used in the USA are not permitted here unless they have been CE marked, and none has. However, Congreve igniter grains are available here, although they are only available for 29mm motors. The lack of legal clarity that prevailed until recently makes alternative methods attractive, if they can be both reliable and cheap.
Damian Hall has developed the pic method, which is very effective and extensively tested for 29mm motors, but is as yet unproven for 38mm and larger motors. It is doubtful whether it will be suited to motors bigger than 38mm, and more work needs to be done to find the limits of this method. We have successfully fired our Sky Ripper J 38mm motor using this method, though - we used two layers of pic, approximately 7 turns in each layer. This requires about 250mm of pic.
We have done some work on using GOx ignition, with both a high-voltage spark and with a low-voltage, high-current system. The former uses a piece of twin-core bell wire, which is cut straight across - no insulator is stripped back, with the gap between the two copper cores being the ideal gap for the spark. The idea is that the spark ignites the wire itself and the fuel grain, sustained by the oxygen. We had problems with the spark box we had, which did not always produce a spark, so we are in the process of making our own from a couple of relays and a car ignition coil. When we have some results they will be posted here.
The second option, using high current, has been tested a little more. The same bell wire is used, but the bared ends are pushed into a bundle of steel wool. A relay connected to the launch controller switches the output from a second battery and puts a large current through the steel wool. Again helped by oxygen, the steel wool burns strongly. However, in our Pentamax 54mm motor we found that the nitrous oxide quenched the wool. We plan to test the method in a 29mm motor shortly. The method has been used to ignite our GOx-supplied lab hybrid, but the remaining steel wool as not ejected from the nozzle, so we are developing a system using butane and a high-voltage spark for this motor.
A new method we have been working on uses magnesium ribbon, which of course burns well once ignited. However, when we tested this inside a motor it extinguished straight away, having consumed all the available oxygen. Now we have a GOx setup, pumping oxygen onto the magnesium means it burns fiercely, and we have been successful in igniting a 29mm RATT Works I-80. We use about 100mm of nichrome resistance wire to heat the magnesium to its ignition point, again connected directly to a 12V battery through a relay. Apart from the setup costs of the oxygen cylinder etc. this is a very cheap method, costing around 25 pence per firing. The next step is to test this method on our 54mm Pentamax motor, and also a 38mm Sky Ripper J motor. More details to follow on this.
We have also had some success using a piece of fuel grain from a 38mm solid motor, used to ignite our 54mm Pentamax. The grain was cut to about 25mm long, and the fill pipe was protected from being burnt too early using a wrap of insulating tape. The grain needs a slow igniter to light reliably, but we have been told that a piece of quickmatch, removed from its outer sleeve, can be used to extend the burn time of a standard Davey match; we haven't tested this. The Pentamax has been tested with various injectors in addition to the standard five-port injector. We have found the AP method to be reliable, but it results in a lot of smoke discharge from the motor. This covers the GSE (and our test stand), as well as the surrounding building and fittings, in black soot.
Costs of the above methods
(prices are in £, pounds sterling)
Standard AP igniter grains from Congreve cost £13.50 for five, suitable for 29mm motors. They come complete with igniters, and can be ignited with the standard igniter connection from any GSE, so there are no setup costs involved.
A piece of AP from a 38mm motor reload, used to ignite our 54mm Pentamax, costs about £4.00, based on making twenty grains from a Congreve I180 reload at £54.00 (two reloads per pack), and purchasing eighteen extra igniters (two are provided with the reload). However, this assumes that all will be used, or the cost per igniter grain goes much higher as the reload is no longer usable for its original purpose.
3mm magnesium ribbon was purchased from eBay, and cost £6.99 for a 25-metre coil, including postage. Making an igniter for a 29mm motor uses about 125mm, giving about 200 pieces from the pack. Nichrome wire is a few pence for the length used, and some 1/8" nylon tubing will also be needed to supply the oxygen, which will be destroyed - again a few pence. (We use flexible nylon pipe rather than a rigid fill stem.) Cost of the oxygen is a few pence as well. Overall, the total cost is estimated at 25 pence per firing. Costs for a 38mm motor are slightly higher, say 30p, and for a 54mm will be about 50p. However, if oxygen is not already available, the cost of setting it up is high. The cylinder must be hired, and costs from about £30.00 per year depending on the size. We have the smallest, and it has lasted a few weeks of reasonably intensive experimentation. It costs less than £3.00 to have it refilled. However, an approved regulator is required, which cost £118.00. It is illegal to play around with the high-pressure side (2000psi), so there is no shortcut here at all. On the discharge side, a solenoid valve, some flexible hose, adapters, a flame arrestor and various pneumatic couplings are needed, and these will cost around £100.00. The total cost of setting up the oxygen system is therefore about £220.00.
For the HT spark and oxygen ignition used by many hybrid fliers, the setup costs for the oxygen are the same, although some motors use a fill stem, which costs £100-150 depending on the size. The spark box costs £50, but is a little cheaper if one can be home-made (we still need to confirm whether our home-made one works reliably). Several fill stems are needed if a range of motors is to be catered for. However, running costs are minimal.
All prices include VAT and are based on 2007 costs.